Metabolic dysfunction is a major contributor to age-related co-morbidities; however the mechanisms underlying muscle metabolic dysfunction have not been fully elucidated in the elderly and there is a need to discover complementary interventions that could be easily and inexpensively implemented for treatment. With a novel strategy to address this knowledge gap, our objective is to investigate synergy between vitamin D repletion and short-term aerobic training (AT) on lipid repartitioning and local muscle oxygen metabolism in the elderly with the use of non-invasive techniques (magnetic resonance imaging (MRI) and hybrid diffuse optical spectroscopies). We suggest based on growing theoretical rationale that a beneficial relationship exists when vitamin D repletion is accompanied by AT in the elderly. A potentiating relationship will prove to have substantial, inexpensive merit for relieving metabolic dysfunction associated with aging. The aims of this study are to 1) Identify the magnitude of lipid redistribution in gastrocnemius muscle following vitamin D repletion + AT compared to vitamin D repletion or AT alone in 65-80 year olds and 2) Quantify local muscle tissue oxygen consumption rate (VO2) using hybrid diffuse optical spectroscopy to elucidate how local muscle tissue metabolism is related to muscle lipid measures. Our central hypothesis is that vitamin D repletion combined with AT will promote a greater shift from EMCL to IMCL depots than either treatment alone and that these changes will correspond to improved oxygen extraction (metabolic function) at the local muscle level. The delineation of the impact of vitamin D repletion combined with AT on maintaining muscle health by altering muscle lipid content using novel and non-invasive techniques has the potential to refine our knowledge of the effect vitamin D and exercise have on muscle biology in aging. The positive impact of this project is consistent with our long-term objectives and are twofold, 1) provide the framework to advance our use of non-invasive diffuse optical spectroscopy to efficiently characterize muscle metabolic function and nutrition/exercise treatment response and 2) generate novel data describing the interrelationships between vitamin D, exercise, muscle lipid and metabolic function in aging muscle.